The present invention relates to a headlight for a motor vehicle having a level for judging whether the angle of the elevational direction of emission of light from the headlight is proper or not. If the angle is judged not to be proper, the headlight can be easily adjusted to make the angle proper.
In a headlight of a type described in U.S. Pat. Nos. 4,794,495 and 4,802,067, a bubble vial type level is attached to a headlight body and reflector unit in which a reflector is integrally formed on the inside curved surface of the body of the headlight. The level extends horizontally in parallel with the optical axis of the headlight. The angle of the direction of emission of light from the headlight can be adjusted by an aiming mechanism while and the graduations of the level, at which an air bubble is located, are observed. There is no problem when the adjustment is performed by observing the graduations of the level while the bulb of the headlight is not lit. However, there is a problem that the position of the air bubble may not accurately indicate the angle of the vertical swing of the level in performing the adjustment while the bulb is lit.
That is, the level of the headlight is attached to a headlight body and reflector unit made of a synthetic resin of low thermal conductivity. The bubble vial and level casing are generally also made of a synthetic resin of low thermal conductivity. Thus, the quantity of heat transmitted from the lit bulb to portions of the bubble vial near the bulb differs from that of heat transmitted from the lit bulb to other portions of the vial farther from the bulb. For this reason, the air bubble of the level is displaced from a proper position which indicates the angle of the vertical swing of the bubble vial.
The present inventors conducted studies and experiments on a level as shown in FIG. 1. As a result, they found out that the reason why the air bubble of the level is displaced from its proper position is that the quantity of heat transmitted from the casing 302 of the level to the bubble vial 304 thereof differs from region to region in the longitudinal direction (the horizontal direction in FIG. 1) of the casing to create a temperature difference in the liquid contained in the vial to cause convection from a region of lower temperature to a region of higher temperature in the vial. The difference in the quantity of the heat results from the fact that a reflector 300 and the casing 302 are made of a synthetic resin which is so low in thermal conductivity that the heat is not uniformly transmitted from a bulb 306 to the whole level. The heat transmitted from the bulb 306 through the reflector 300 to the front end portion 302a of the casing 302, which is nearer the bulb, is accumulated, making the temperature of the portion as high as about 79.degree. C., because the reflector and the casing are low in thermal conductivity. The heat is not well transmitted from the bulb 306 through the reflector 300 to the rear end portion 302b of the casing 302, which is farther from the bulb, so that the temperature of that portion is lower (at about 70.degree. C.) than that of the front end portion 302a of the casing, because the reflector and the casing are low in thermal conductivity and the heat transmission passage from the bulb to the rear end portion is longer than that from the bulb to the front end portion.
A temperature difference of about 9.degree. C. thus occurs between the front and rear end portions 302a and 302b of the casing 302, so that a temperature difference of about 8.degree. C., nearly equal to the former, is developed between the front and rear end portions of the bubble vial 304. Convection is caused in the vial 304 due to this temperature difference, as a result of which the air bubble 305 is displaced from its proper position. The maximum displacement of the air bubble 305 corresponded to a swing angle of 0.38 degree in the inventors' experiments.
Accordingly, it was determined that if the thermal conductivity of the casing 302 of the level is increased, the quantity of the heat transmitted from the bulb 306 to the region of the casing 302 farther from the bulb is nearly equalized to the amount of the heat transmitted from the bulb to the other regions of the casing nearer the bulb, so that the quantities of the heat transmitted to the respective regions of the bubble vial 304 through the casing are nearly equalized to each other, namely, the difference between the temperatures of the regions of the vial is reduced. The present invention is based on this determination.